1. Field of the Invention
The present invention relates to improvements in unsteady-state fluid flow regimes and to their industrial applications.
2. Description of the Prior Art
In industrial installations for conveying compressible fluids, it is a common practice to carry out fluid expansions through orifices located between enclosures at different pressures without recovery of kinetic energy.
This is the case in particular with expansion valves and multi-bore pressure reducers or screen tubes such as those employed in steam power plants.
The mass flow rate as well as the upstream and downstream pressure levels are mostly imposed and the pressure ratio is often sufficiently high to permit the establishment of supersonic flow regimes.
The general principle of devices of this type lies firstly in acceleration of the fluid flow to full velocity followed by degradation of said velocity as a result of viscous friction forces and shock waves in a constant evolutionary process of stagnation enthalpy.
In a flow discharge from an orifice into the surrounding atmosphere or into a large enclosure, the flow pattern at the outlet of the orifice is different according to the expansion ratio. Nevertheless, the conditions of pressure on each side of the orifice are usually such that the flow in a first estimation is sonic in the outlet plane of the orifice.
As a result of viscous mixing with the surrounding fluid, the jet will deteriorate after a distance which will be variable according to the expansion ratio, this distance being usually greater than ten times the diameter of the orifice.
In the case of a two-dimensional flow discharge from an orifice into a downstream cavity, the supersonic jet comes into adhesive contact with either one wall or the other in a somewhat abrupt manner but without producing an unsteady state.
A viscous entrainment phenomenon in fact causes slowing-down of the jet and acceleration of the peripheral fluid layers. The entrained mass is renewed by fluid delivered at the downstream end. Since there is a state of independence between the mixture layers, the smallest disturbance destabilizes the system.
When the downstream pressure is reduced, the jet diverges to such an extent that the cavity which has remained in communication with the downstream end can no longer be supplied with mixture fluid, whereupon longitudinal and transverse oscillations of the fluid stream are observed. If a further pressure drop takes place from this position onwards, the flow stream is restored to a steady state and becomes symmetrical. Under conditions of flow within a cavity, similar observations are made but are more complex and difficult to analyze since the jet flows in rotational or swirling motion within the cavity.
Flow discharge from a plurality of orifices in parallel into a large enclosure or vessel is utilized systematically in the industrial field. This method is justified by the resultant increase in friction surfaces on the downstream side of the orifices inasmuch as loss by abrupt throttling and fluid-wiredrawing within the orifice is usually minor in comparison with the residual velocity loss.
Expansion is arrested as soon as the jet flow lines meet each other and the general effect thus produced is the same as if the flow had emerged from a single orifice, thus expanding to the pressure at which the above-mentioned jet flow lines meet, with the result that the friction surface area is abruptly reduced. It should be added that abrupt increases in length of the jet also occur in this case.
It may thus be stated that, up to the present time, the known techniques do not make it possible to ensure satisfactory mixing between the jets and the surrounding fluid and also fail to ensure steady-state stability of the fluid stream.
Return flows along the walls are therefore not capable of supplying mixture fluid to supersonic jets which accordingly undergo degradation only through oblique or straight shock-wave systems which are more or less stable at the time of initiation of the stream flow.
It is illusory to expect that jets can be mixed at similar flow velocities.
Finally, unstable flow regimes produce strong vibrations which cause degradation of structures and especially valves or pressure reducers.